The long term goal of the proposed studies is to determine the rules that govern meiotic recombination (gene conversion, crossing-over and post- meiotic segregation (PMS) ) in the yeast Saccharomyces cerevisiae. Most of the proposed experiments concern recombination at the H1S4 locus (which has an extremely high rate of recombination in some lab strains). There are six specific aims. First, the DNA sequences responsible for initiating gene conversion events at the H1S4 locus (and elsewhere) will be identified by deletion analysis. Second, the direction in which gene conversion events are propagated form the initiation site will be investigated by examining the conversion rates of genes flanking H1S4 in strains in which the conversion initiation site of H1S4 has been deleted. Third, the DNA sequences that are responsible for high PMS alleles will be determined by using in vitro mutagenesis procedures to generate H1S4 mutant alleles at the initiating AUG codon. Fourth, genetic methods will be developed to examine which DNA strand (coding or non-coding) of H1S4 is transferred during heteroduplex formation. Fifth, the DNA lesions (single-strand nicks or double-strand breaks) that initiate recombination will be studied by gel electrophoresis of meiotic DNA. Sixth, the chromatin structure of recombination hotspots will be investigated (by indirect end-labeling techniques) in order to determine whether these sequences are accessible to nucleases. Since predictions concerning the likelihood of individuals having a genetic defect are often based on meiotic genetic linkage analysis, it is important to understand the basic properties of meiotic recombination.